The present invention pertains to the art of gas turbine engines and, more particularly, to a system for delivering air from a multi-stage compressor to a turbine portion of a gas turbine engine to enhance turbine engine performance during low turn down and/or operation below design temperatures.
Turbine engines are designed to operate at full speed, International Organization for Standardization (ISO) conditions. That is, turbine engines are designed to utilize a defined range of input conditions, e.g., design point temperature, barometric pressure, fuel type etc, to operate within a defined speed band to produce an optimal, high, output below which operation is less than efficient. Often times however, input conditions are outside the design points, and/or a lower than full output is desired. During ISO operation, air is fed into a combustor to mix with fuel to form a high pressure, high temperature combustion product that is delivered to the turbine. At full speed, the combustor requires a high volume of air in order to produce enough combustion products to drive the turbine. In addition, various components of the turbine, such as rotor blades, require cooling.
Turbine component cooling circuits use sourced air from a compressor stage that delivers adequate pressure across a majority of an overall operating range of the gas turbine. Most combustion turbines use air that is bled from one or more compressor extraction stages of an integral compressor component to provide for cooling and sealing in the turbine component. Air extracted from the compressor for this purpose may be routed internally through passages formed in a compressor-turbine rotor assembly, or the like, to locations that require cooling and sealing in the turbine component. Alternatively, air may be routed externally through piping that extends between the compressor component and the turbine component.
Extraction air having sufficient pressure during optimal operating conditions often times does not have enough pressure during less than optimal conditions such as when operating below the design point temperature and/or during turn down when output is reduced. In order to provide sufficient pressure during the less than optimal conditions, primary extraction air is supplemented with additional, high pressure, extraction air from a high compressor stage. Unfortunately, while air from a higher stage adds to the cooling air pressure, the use of high pressure air is very inefficient or detrimental to the overall energy producing process. That is, more work is input into forming high pressure air than is required to produce lower pressure extraction air. For this reason, supplemental air is typically limited to mid level compressor stages. If supplemental air is taken from a higher compressor stage, the high stage air is only combined with a medium primary stage extraction air in order to minimize any pressure differential between the extraction stages thereby such that only a small portion of the high stage extraction air is required. In this manner, engine operation remains cost effective during cold days and turn down periods.